The common trend with valveless down-the-hole hammers is to have a drill bit with a replaceable foot valve tube protruding from the head of the bit at the striking face of the bit and piston. This foot valve tube extends into the bore of the piston and when the piston moves away from the bit the foot valve tube pulls out of the bore of the piston. Exhaust from the lifting or lower chamber takes place through the center of the foot valve tube to the atmosphere via the center bore of the bit.
While the piston is striking the bit, and the bore of the piston is engaged with the foot valve tube, the upper chamber is exhausting through the bore of the piston through the center of the foot valve tube and out to the atmosphere via the bore of the bit.
Exhaust from the lower and upper chamber thus takes place alternately during operation as the piston reciprocates under influence of the pressurized working fluid.
The foot valve tube is normally replaceable with one end fixed in the body of the bit and the other end is a sliding fit into the bore of the piston. The outer diameter of the foot valve tube is limited as an increase in diameter of the foot valve tube reduces the lifting surface area of the piston. It will also weaken the walls of the piston and bit at its striking face. To increase the strength of the foot valve tube the walls have to be made thicker. This will cause a further restriction in the exhausting air of the hammer and will result in reducing the performance of the hammer. Another problem experienced is the frequent breakage of the foot valve tube. This results in down time for the driller as he has to pull the drill string out the hole, remove the bit from the hammer and replace the foot valve tube.
A drill hammer assembly having an upper and lower chamber as well as a foot valve tube is disclosed in the applicant's earlier filed U.S. Pat. No. 6,543,557.